The present invention relates to fluorescent lamps having multi-layer phosphor coatings, and more particular to such lamps optimized with respect to lumen output, color rendering and cost.
Fluorescent lamps in which the fluorescent material is a halophosphate are well known, and such lamps have been highly developed. These lamps have the virtue of being relatively inexpensive, and at the same time having a good luminous efficacy and an acceptable color rendering index (CRI). The halophosphates are not suitable for use in small diameter fluorescent lamps in which the phosphor layer is close to the ionized mercury in the discharge stream.
The unit of lamp diameter is 1/8 inch. Conventional tubular fluorescent lamps have an outer diameter of 1.5 inches and are referred to as T12 lamps. In recent years, narrower diameter T10 lamps, and even T8 lamps have become increasingly popular. In these narrower diameter lamps the phosphor layer is closer to the discharge stream of mercury ions and degrades more quickly over time, thus shortening lamp life. Halophosphate phosphors are generally less satisfactory in T10 and T8 lamps than in T12 lamps.
Another class of phosphors having increasingly broad applications in recent years are the rare earth activated phosphors. Fluorescence in these phosphors is characterized by relatively narrow emission bands. A fluorescent lamp having a rare earth activated phosphor, with emission in the red, green and blue portions of the spectrum has the potential for achieving very good color rendering characteristics. Such a lamp is disclosed in U.S. Pat. No. 3,937,998 (Verstegen et al), and can readily achieve CRI values in excess of 80.
Rare earth activated phosphors, such as aluminates, also have high luminous efficacy. Moreover, these phosphors readily withstand the mercury ion discharge stream in fluorescent lamps. They are thus suitable for narrow diameter fluorescent lamps, even narrower than T8. A disadvantage of such phosphors is that they are relatively expensive.
Lamps having both halophosphate phosphors and rare earth activated phosphors are also known. In these lamps, the halophosphate phosphor is disposed directly on the lamp envelope, and then an overlying rare earth activated phosphor layer is disposed on the halophosphate. If the rare earth activated phosphor is of a type that can withstand interaction with mercury ions better than the halophosphate, for example the aluminates, the rare earth activated phosphor layer may act as a protective covering to reduce halophosphate degradation. In addition, the rare earth phosphors result in lamps having a higher CRI than lamps made with just a halophosphate.
An example of such a multi-layer phosphor lamp is disclosed in U.S. Pat. No. 4,751,426 (Hoffman et al). That patent discloses a fluorescent lamp having a halophosphate layer which emits light having a color temperature falling within the standard cool white oval of the CIE chromaticity diagram. A second layer of rare earth activated phosphors covers the halophosphate layer and improves the CRI of the light emitted by the lamp.
It would be desirable to minimize the amount of rare earth activated phosphor in the lamp in order to reduce cost. The amount of rare earth phosphor can not be reduced arbitrarily, however, or the lamp CRI may be unacceptably low. Additionally, the rare earth activated phosphor must form a continuous layer covering the halophosphate. This is especially important in smaller diameter lamps because a function of the rare earth activated phosphor layer is to protect the underlying halophosphate layer from degradation by the mercury ion stream.
Accordingly, it is an object of the invention to provide a multi-layered fluorescent lamp having an acceptable CRI and optimized to maximize lamp efficacy and lower cost.